Hot melt adhesives are solid at room temperature but, upon application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. Curable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. An advantage of hot melt curable adhesives over traditional liquid curing adhesives is their ability to provide “green strength” upon cooling prior to cure. Advantages of hot melt curable adhesives over non-curing not melt adhesives include improved temperature and chemical resistance.
The majority of reactive hot melts are moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer. Polyurethane prepolymers are conventionally obtained by reacting polyols with isocyanates. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the adhesive, and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a crosslinked material polymerized primarily through urea groups and urethane groups.
Additives are commonly included in reactive hot melt adhesive formulations. It is particularly advantageous to incorporate low cost additives that would provide improved properties. Any additives, however, must be compatible with the adhesive so as to provide a smooth textured adhesive. Incompatible additives result in undesirable properties, such as graininess in the adhesive's texture and poor shelf stability. Bituminous materials are often low in cost, however have not been successfully utilized in reactive hot melt adhesives because of many undesirable properties, such as lack of compatibility. It would be desirable to develop a reactive hot melt adhesive that utilized a low cost bituminous additive with desirable results, such as a non-objectionable odor and storage stability. The present invention addresses this need.